Optical disks widely used in recent years include, for instance, data read-only disks such as CD and CD-ROM, so-called rewritable optical disks such as CD-R, as well as magneto-optical disks called MO or MD. The recording capacity of these disks is about 650 MB (megabytes). Disks having greater capacities such as a family of optical disks called DVD (digital versatile disk) have also been developed. Specific examples of DVD include DVD-ROM (read only DVD), DVD-R (write-once DVD), DVD-RAM (write/read DVD) and DVD-RW (rewritable DVD). DVD types in practical use include types comprising two substrates bonded to each other, and have a recording capacity of 4.7 GB (gigabytes), on one side, and 9.4 GB on both sides. Red laser light of a wavelength ranging from about 630 to 650 nm is used for data recording and reproduction.
In case, for instance of high-definition video, current DVD types are however problematic in that recording is limited to only about 30 minutes on one side. It would be desirable for one side to be capable of recording at least two hours of high-definition video, to which end large-capacity optical disks are required. In order to implement such optical disks, recording/reproduction must be carried out using laser light of a shorter wavelength.
Accordingly, Blu-ray disk (BD) relying on blue laser light having a wavelength of about 405 nm is currently being implemented. Using blue laser light of such a short wavelength allows reducing both track pitch and pit size. However, the shorter wavelength is problematic in that it entails a shallower focus depth, which precludes the use of specifications and methods currently employed in the DVD family, namely bonding of substrates having each a thickness of 0.6 mm (to a total substrate thickness of 1.2 mm).
Therefore, for instance, a data recording layer provided on a 1.1 mm-thick substrate has bonded thereon a 0.1 mm-thick light-transmitting protective film of the same material as the substrate. Examples of methods for bonding the protective film in this case include, for instance, using an energy-beam curable adhesive or pressure-sensitive adhesive sheet. Such energy-beam curable adhesives and adhesive sheets must not impair the recording/reproduction function of the optical disk.
In a method of bonding a protective film using an energy-beam curable adhesive, the adhesive is ordinarily coated by spin coating, which is problematic in that thickness unevenness is likely to occur. By contrast, bonding using a pressure-sensitive adhesive sheet allows realizing high thickness precision.
Such optical disks are exposed to harsh environments in actual use. When used on board vehicles, for instance, the temperature can be anticipated to rise to about 80° C. in midsummer, and to drop to about −20° C. in cold regions. During rainy seasons, moreover, the optical disk may be exposed not only to high temperature, but also to high humidity. Depending on the type of the pressure-sensitive adhesive, the metal layer/metal oxide layer of the data recording layer may corrode when the optical disk is used under such conditions. Also, changes in temperature may result in delamination between recording layers, between a recording layer and the pressure-sensitive adhesive, or between the protective film and the pressure-sensitive adhesive, as a result of which the functionality of the optical disk may be lost.
A possible method of enhancing adhesion to metal layers/metal oxide layers involves ordinarily incorporating acidic groups into the main component of the pressure-sensitive adhesive. However, this gives rise to corrosion of the metal layer/metal oxide layer, in particular at high temperature and high humidity. Corrosion is suppressed if acidic groups are absent, but, conversely, adherence drops dramatically.
In order to solve the above problems, Patent document 1 proposes a pressure-sensitive adhesive based on a (meth)acrylate copolymer that contains a (meth)acrylate and a nitrogen atom-containing copolymerizable monomer but that contains no carboxyl group-containing copolymerizable monomer.
Patent document 1: Japanese Patent Application Laid-open No. 2005-325250
However, Patent document 1 does not address the behavior of the properties of the aforementioned (meth)acrylate copolymer with respect to temperature. Actually, the features disclosed in the examples result in delamination during heat cycle testing according to the envisaged use environment. This can arguably be ascribed to insufficient adhesion of the pressure-sensitive adhesive to the metal layer/metal oxide layer, and to stretching and shrinkage of the pressure-sensitive adhesive layer. That is, the adherence and delamination prevention required in practice fail to be achieved by merely using a monomer component in the form of a (meth)acrylate copolymer having a nitrogen atom-containing copolymerizable monomer.